The present invention relates to a motor employing a ball bearing, more particularly relates it to a bearing structure of the motor, where unbalance load in a radial direction or thrust force in an axial direction is applied to the bearing.
Recently, a motor of industrial-use is not only used for driving a conveyor and the like, but also used in variety of applications such as a robot or a pump, and the motors used in those machines work continuously for long hours. The market requires a ball bearing employed in those motors to have a longer service life because eccentric load in a radial direction or thrust force in an axial direction is applied to a motor shaft.
A motor with a ball bearing uses a wave washer in order to apply appropriate pre-load to the ball bearing, so that a longer service life of the ball bearing can be expected. A little space is provided between an outer ring of the ball bearing and a housing of the bearing to make the assembly a bit loose for pre-loading the ball bearing in the axial direction and for absorbing dimensional tolerance of the parts used in the motor.
A bearing structure of a conventional motor, which is employed for driving a conveyor, is described hereinafter with reference to FIG. 4. In FIG. 4, rotor iron-core 17 is rigidly fit to motor shaft 16, and stator iron-core 19 with coils is rigidly fit to frame 18. Rear bracket 20 and front bracket 21 are mounted to an opening of frame 18, and each of the brackets has a bearing housing at its center. Each of inner rings of rear ball bearing 22 and front ball bearing 23 is rigidly fit to motor shaft 16, and an outer ring of rear ball bearing 22 is rigidly bonded to the bearing housing disposed at rear bracket 20. An outer ring of front ball bearing 23 is not bonded. Wave washer 24 is inserted in front of bearing 23, so that the ball bearing is pre-loaded by the spring pressure.
The structure discussed above, however, invites creep to the outer ring of the front ball bearing because the outer ring is simply left loose and pre-loaded by the wave washer. In particular, when unbalance load is applied in the radial direction, the outer ring becomes more subject to the creep, thereby reducing the service life of the bearing.
In the case of a servo-motor equipped with an optical encoder at its shaft, when the shaft moves in the axial direction against the pre-load by the wave washer, a rotary section and a stationary section of the encoder come into contact with each other, which may damage the encoder.
The present invention addresses the problems discussed above, and aims to provide a motor, of which ball bearing is prevented from creep at its outer ring, usable in variety of applications.
A motor of the present invention comprises the following elements:
a cup ring disposed before a front bracket; and
a holding plate disposed behind the front bracket.
The cup ring and the holding plate sandwich the front bracket, and the cup ring and the plate are screwed down to the front bracket so that the outer ring of the front ball bearing is tightly sandwiched and cannot move. As a result, this structure prevents the outer ring from incurring creep.